A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to obtain a high accuracy and high resolution currently aimed at in lithography, it is desirable to accurately position parts of the lithographic apparatus such as the reticle stage to hold the patterning device (e.g. mask), the projection system and the substrate table to hold the substrate, with respect to each other. Apart from the positioning of e.g. the patterning device (e.g. reticle) stage and the substrate table, this may also pose requirements on the projection system. The projection system in current implementations may consist of a carrying structure, such as a lens mount (in case of transmissive optics) or a mirror frame (in case of reflective optics) and a plurality of optical elements such as lens elements, mirrors, etc. In operation, the projection system may be subject to vibrations due to a plurality of causes. As an example, movements of parts in the lithographic apparatus may result in vibrations of a frame to which the projection system is attached, a movement of a stage such as the substrate stage or the patterning device (e.g. reticle) stage, or accelerations/decelerations thereof, which may result in a gas stream and/or turbulence and/or acoustic waves affecting the projection system. Such disturbances may result in vibrations of the projection system as a whole or of parts thereof. By such vibrations, displacements of lens elements or mirrors may be caused, which may in turn result in an imaging error, i.e. an error in the projection of the pattern on the substrate.
Commonly, a damping system is provided to dampen vibrations of the projection system or parts thereof. Thereto, a passive damping system may be provided as known in many forms, or an active damping system, or a combination of a passive and a active damping system. In this document, the term active damping system is to be understood as a damping system which includes a device to detect or determine an effect of a vibration (e.g. a position sensor, velocity sensor, acceleration sensor, etc) and an actuator to act on the structure to be damped or a part thereof, the actuator being driven by e.g. a controller in dependency of a signal provided by the sensor. By driving the actuator in dependency of the signal provided by the sensor, an effect of the vibration on the projection system or a part thereof, may be reduced or cancelled to a certain extent. An example of such active damping system may be provided by a feedback loop; the sensor provides a position quantity (such as a position, speed, acceleration, jerk, etc of the projection system or a part thereof), which is fed into a controller, the controller generates a controller output signal to drive the actuator, and the actuator in turn acts on the projecting system or the part thereof so that a feedback loop is provided. The controller may be formed by any type of controller and may be implemented in the software to be executed by a microprocessor, microcontroller, or any other programmable device, or may be implemented by dedicated hardware.
It is desirable to obtain stability of the feedback loop, i.e. to achieve a frequency behavior of the feedback loop wherein ringing and/or oscillation is prevented. At the same time, a high bandwidth of the active damping system is desired, as a high bandwidth of the active damping system will allow to suppress vibrations within such high bandwidth. Due to the ever increasing demands on speed of the lithographic apparatus, movements in the lithographic apparatus tend to take place at a higher speed and consequently involving faster transients, which may result in a generation of vibrations at increasingly higher frequencies. Therefore, a demand comes forward towards a higher bandwidth of the active damping system.
A phenomenon that is encountered is that the projection system is commonly built up from a variety of parts, including e.g. lenses, mirrors and/or other optical elements, lens mountings and/or mirror mountings, a housing of the projection system such as a lens body, etc. As a consequence, a frequency behavior of the projection system starts, at a low frequency extreme, as a rigid body mass, thereby providing a transfer function from a force acting on the projection system to a velocity of the projection system which is inversely proportional to a frequency, assuming that the frame on which the projection system is mounted is already decoupled from the fixed world. In a resonance frequency range, a resonance of the projection system is observed, which may be followed by a plurality of further resonances with increasing frequency, thereby overall resulting in an increase of the magnitude of the transfer function. Effectively, as from the resonance frequency range, the projection system does not behave as a single object anymore, however instead shows a variety of resonance phenomena each corresponding to resonance of an element of the projection system. As a result thereof, the higher the frequency, the lower the remaining mass which “contributes” to the transfer function, which may be considered an explanation for the fact that the magnitude of the transfer function from a force acting on the projection system to a velocity of the projection system increases with increasing frequency, in the frequency range above the resonance frequency range.
As will be understood by a skilled person, the frequency behavior of the projection system as outlined above, may result in stability problems when attempting to achieve a bandwidth of the active damping system high enough to dampen resonances which reaches or exceeds the resonance of the projection system. The transfer function may be expressed in terms of e.g. velocity of the projection system as a function of a force on the projection system. It is noted that the transfer function may also be expressed in any other suitable quantity, such as acceleration of the projection system as a result of force on projection system. In that case, a low frequency behavior of the transfer function will show to be frequency independent, followed by a resonance frequency range and an increase of the transfer function (showing multiple resonance peaks) above the resonance frequency range.
A sensor is used in an active damping system to determine the position quantity of the projection system, typically the acceleration of the projection system. The quality of the sensor signal has to be high in order to obtain an optimal damping performance. However, the application of the sensor has some practical drawbacks.
Normally, the sensor may not be placed in one line with the actuator and will therefore be placed next to the actuator. As a result, it may not be perfectly collocated with the actuator. Due to this non-perfect collocation, the damping performance may be less.
Furthermore, the sensor, for instance a mass coupled to a piezo sensor, is generally capable of measuring the acceleration in a certain frequency range with high accuracy, but performance may become worse in other frequency ranges. Moreover, the sensor may include resonance frequencies in a relevant frequency range. In such case, the resonance frequencies may become determining for the achievable gain, and thus the damping performance.
Sensors with relative low resonance frequencies have typically a good performance in a low frequency range but limit the bandwidth of the active damping system. Sensors having relative high resonance frequencies perform typically well for higher frequencies and have correspondingly a higher bandwidth. However, performance for lower frequencies is relatively bad.
It is remarked that actuators used for an active damping system may have the same characteristics, i.e. actuators with lower resonance frequencies perform well for lower frequencies but have limited bandwidth, while actuators with higher resonance frequencies provide higher bandwidth but have lower performance for low frequencies.